Marine oils used in slow two-stroke cross-head engines, are of two types. Cylinder oils on the one hand ensuring lubrication of the piston-cylinder assembly, and system oils on the other hand ensuring lubrication of all the moving parts except for the piston-cylinder assembly. Within the piston-cylinder assembly, combustion residues containing acid gases are in contact with the lubricating oil.
Acid gases are obtained from the combustion of fuel oils; these are notably sulfur oxides (SO2, SO3), which are then hydrolyzed upon contact with moisture present in the combustion gases and/or in the oil. This hydrolysis generates sulfurous acid (HSO3) or sulfuric acid (H2SO4).
In order to preserve the surface of the cylinder sleeves and to avoid excessive corrosive wear, these acids have to be neutralized, which is generally carried out by reaction with basic sites included in the lubricant.
The neutralization capacity of an oil is measured by its BN or Base Number characterizing its basicity. It is measured according to the ASTM D-2896 standard and is expressed in potash weight equivalents per gram of oil or mg of KOH/g. BN is a standard criterion allowing adjustment of the basicity of cylinder oils with the sulfur content of the fuel oils used, in order to be able to neutralize the totality of the sulfur contained in the fuel, and capable of being transformed into sulfuric acid by combustion and hydrolysis.
Thus, the higher the sulfur content of a fuel oil, the higher should be the BN of marine oil. This is why marine oils with BN varying from 5 to 100 mg KOH/g are found on the market.
Environmental concerns have induced in certain areas notably in coastal areas, requirements as regards limitation of the sulfur level in fuel oils used on ships.
Thus, the MARPOL Annex 6 regulations (Regulations for the Prevention of air pollution from ships) of the IMO (International Maritime Organization) came into force in May 2005. It foresees a maximum sulfur content of 4.5% m/m for heavy fuel oils as well as the creation of areas with controlled emission of sulfur oxides, called SECAs (Sox Emission Control Areas). Ships entering these areas will have to use fuel oils with a maximum sulfur content of 1.5% m/m or any other alternative treatments aiming at limiting Sox emissions in order to observe the specified values. The notation % m/m refers to the mass percentage of a compound based on the total weight of fuel oil or lubricant composition in which it is included.
Ships covering transcontinental routes will then use several types of heavy fuel oils depending on the local environmental constraints and this allows them to optimize their operational costs.
Thus, most container carrier ships presently being built foresee the application of several bunkering tanks, for ‘high sea’ fuel oil with a high sulfur content on the one hand and for ‘SECA’ fuel oil with a sulfur content of less than or equal to 1.5% m/m on the other hand.
Switching between both fuel oil categories may require adaptation of the operating conditions of the engine, in particular the application of suitable cylinder lubricants.
Presently, in the presence of high sulfur content fuel oil (3.5% m/m and more) marine lubricants are used having a BN of the order of 70.
In the presence of a low sulfur content fuel oil (1.5% m/m and less), marine lubricants are used having a BN of the order of 40.
In both of these cases, a sufficient neutralization capacity is then attained since the required concentration of basic sites provided by the overbased detergents of the marine lubricant is reached, but it is necessary to change the lubricant at each change in the type of fuel oil.
Further, each of these lubricants has limits of use resulting from the following observations: the use of a BN 70 cylinder lubricant in the presence of a fuel with a low sulfur content (1.5% m/m and less) and a set greasing level, creates a significant excess of basic sites (strong BN) and a risk of destabilization of the unused overbased detergent micelles, which contain insoluble metal salts. This destabilization results in the formation of deposits of insoluble metal salts (for example calcium carbonate), mainly in the piston junk, and may eventually lead to a risk of excessive wear of the cylinder sleeve polishing type.
Consequently, optimization of the lubrication of a slow 2-stroke engine then requires selection of the lubricant with the BN adapted to the fuel oil and to the operating conditions of the engine. This optimization reduces the flexibility of operating the engine and requires great technical skill of the crew in defining conditions under which switching from one type of lubricant to the other should be performed.
In order to simplify the maneuvers, it would therefore be desirable to have a single cylinder lubricant for a 2-stroke marine engine which may be used both with high sulfur content fuel oils and with low sulfur content fuel oils.